Beyond Planetary Nebulae

Last week I accounted for the evolution of stars like our Sun, and up to 8 times its mass. The end game is a white dwarf, most likely with spherical shells of gas and debris flowing away. Intense radiation from the white dwarf causes the dust and gas to glow, making it look telescopically like a planet in many cases. It’s called a planetary nebula and they are beautiful to observe and photograph.

But, what about stars with mass beyond 8 times our Sun’s mass? How do they evolve and what do they become?

We’ll look at stars between 10 and about 25 times the mass of our Sun.

Stars beyond 10 times the mass of our Sun live fast and die young. Well, at least younger than stars like our Sun. These massive stars consume hydrogen a lot faster and are hotter than the Sun. They tend to go through their hydrogen stores in hundreds of millions of years to a few billion years. The evolution of fusion progresses rapidly, through helium, carbon, oxygen, eventually to iron. Iron is fusion’s wall. Iron fusion is not sustainable, it takes more energy to fuse iron than the fusion produces.

Once iron fusion begins the star’s core collapses as outward energy pressure drops precipitously and, well, gravity just wins. At this point the star’s mass is still beyond 8 times that of the Sun and that much mass produces nearly irresistible gravitational force. The meager energy from the iron fusing core is in for a pounding and this pounding happens with a flash! There is no pulsing as with Sun like stars, no puffing off shells of gas and dust. Just a rapid core collapse until degeneracy pressures (see white dwarfs, electrons do not like to share the same space) stops it, but only briefly. Its core increases in mass by in-falling stellar debris. There is so much mass (gravity) electron degeneracy pressure cannot hold the collapse very long, this star cannot form a white dwarf.

The core collapses past electron degeneracy, crushing them together and heating the core beyond 5 billion degrees K. A phenomenon called photodisintegration occurs where iron nuclei in the core break up, becoming alpha particles. Temperatures continue to climb, neutrons are produced from protons through a process called electron capture. Things are getting crazy! Electron capture becomes a runaway chain reaction, and a massive outward flux of neutrinos kicks all the in-falling material away in a supernova!

All that remains is the tiny core, the next level of stellar evolution, a neutron star. But is it a star? Neutron stars do not produce energy – the definition of a star. Should they be called neutron dwarfs, maybe blue dwarfs? They are hotter than white dwarfs, but will eventually cool…into what?

More next week.

What’s in the Sky?

April 18; 45 minutes before sunrise; east-southeast: Jupiter, Venus, Mars, and Saturn are lined up for a family photo.

April 22; 1am-dawn; southeast: The Lyrid Meteor Shower peaks. A bright Moon interferes.